Nitride semiconductors having high breakdown electric field and high saturation electron velocity have been attracting attention as the next generation of semiconductor materials for high-frequency/high-power devices. For example, a high electron mobility transistor (HEMT) device formed by laminating a barrier layer composed of AlGaN and a channel layer composed of GaN takes advantage of the feature that high-concentration two-dimensional electron gas (2DEG) is generated at a lamination interface (hetero interface) owing to a polarization effect (spontaneous polarization effect and piezo polarization effect) inherent in a nitride material (for example, see “Highly Reliable 250 W High Electron Mobility Transistor Power Amplifier”, Toshihide Kikkawa, Jpn. J. Appl. Phys. 44 (2005), p. 4896).
As a base substrate of the substrate for HEMT device, for example, a single crystal (heterogeneous single crystal) having a composition different from that of a group III nitride, such as silicon and SiC, is used in some cases. In this case, a buffer layer such as a strained superlattice layer and a low-temperature growth buffer layer is typically formed as an initial growth layer on the base substrate. Therefore, the most basic configuration of a substrate for HEMT device using a base substrate formed of heterogeneous single crystal is obtained by epitaxially forming a barrier layer, a channel layer and a buffer layer on a base substrate. In addition, for the purpose of accelerating spatial confinement of two-dimensional electron gas, a spacer layer having a thickness of approximately 1 nm is provided between the barrier layer and the channel layer in some cases. The spacer layer is composed of, for example, AlN. Moreover, for the purposes of controlling an energy level on the topmost surface of the substrate for HEMT device and improving contact characteristics with an electrode, for example, a cap layer composed of an n-type GaN layer or a superlattice layer is formed on the barrier layer in some cases.
In a case of a nitride HEMT device having the most typical configuration in which a channel layer is formed of GaN and a barrier layer is formed of AlGaN, it is known that the concentration of two-dimensional electron gas existing in a substrate for HEMT device increases along with an increase in AlN mole fraction of AlGaN that forms the barrier layer (for example, see “Gallium Nitride Based High Power Heterojunction Field Effect Transistors: Process Development and Present Status at USCB”, Stacia Keller, Yi-Feng Wu, Giacinta Parish, Naiqian Ziang, Jane J. Xu, Bernd P. Keller, Steven P. DenBaars, and Umesh K. Mishra, IEEE Trans. Electron Devices 48 (2001), p. 552). It is conceivable that controllable current density of a HEMT device, that is, power density capable of being utilized can be improved significantly if the concentration of two-dimensional electron gas can be increased significantly.
Further, growing attention is also paid to the HEMT device that has a low dependence on the piezo polarization effect, is capable of generating two-dimensional electron gas at high concentration almost only by spontaneous polarization, and has the structure with small strains, such as the HEMT device in which a channel layer is formed of GaN and a barrier layer is formed of InAlN (for example, see “Can InAlN/GaN be an alternative to high power/high temperature AlGaN/GaN devices?”, F. Medjdoub, J. -F. Carlin, M. Gonschorek, E. Feltin, M. A. Py, D. Ducatteau, C. Gaquiere, N. Grandjean, and E. Kohn, IEEE IEDM Tech. Digest in IEEE IEDM 2006, p. 673).
In order to put the above-mentioned HEMT device or a substrate for HEMT device that is a multi-layer structure used in manufacturing the same to practical use, various problems need to be solved; problems related to performance improvement such as increase of power density and efficiency, problems related to functional improvement such as achieving normally-off operation, and fundamental problems such as enhancing reliability and reducing cost. The above-mentioned problems are individually tackled vigorously.
One of the above-mentioned problems is to improve schottky contact characteristics between a gate electrode and a barrier layer.